Aminoplast crosslinker resins, particularly optionally etherified resins derived from melamine and formaldehyde, have been used in many industrial coating applications on substrates such as wood and metals, for fields such as automotive, appliances, and coil coating, as crosslinkers to form coating films comprising a reticulated three-dimensional polymer network. Functional groups of aminoplast crosslinker resins can react with functional groups of the binder resins, particularly hydroxyl groups, as well as with themselves in a self-condensation reaction. Aminoplast crosslinker resins are usually mixtures of monomeric and oligomeric molecules.
In the case of melamine formaldehyde resins, the melamine unit comprises a heterocyclic six-membered triazine ring of alternating carbon and nitrogen atoms where each of the three ring carbon atoms has a pendant nitrogen atom which is not part of the triazine ring and can link to two functional groups, by substitution of the hydrogen atoms bound to these nitrogen atoms in melamine. Thus, each monomeric melamine unit has six functional groups. In the synthesis of melamine and formaldehyde based amino crosslinker resins, melamine is reacted with formaldehyde to form a mixture of different methylol melamines, where at least one molecule of formaldehyde is reacted under addition to the aminic nitrogen atom to form a so-called N-methylol group of the formula >N—CH2—OH, up to hexamethylol melamine where all of the aminic nitrogen atoms carry two methylol groups. Reaction of more than six formaldehyde molecules is also possible, under formation of structures such as >N—CH2—(O—CH2)n—OH, with n being 0 for a N-methylol group, or larger that 0, i. e. 1 or more where oligoacetal groups are formed. The methylol compound may then be reacted with an aliphatic alcohol, preferably methanol, n-butanol, or isobutanol, or mixtures of these, to form a mixture of etherified methyolmelamines, e.g., in a stoichiometry of 1 mol of melamine combined with 6 mol of formaldehyde and 6 mol of the aliphatic alcohol, R—OH, hexa-alkoxymethyl melamine where each of the aminic nitrogen atoms carry two alkoxymethyl groups —N(CH2—OR)2.
The reaction of melamine with formaldehyde and alcohols can thus provide a variety of functional groups attached to the pendant nitrogen atoms including groups which represent bridges between triazine units. These functional groups include imino (>N—H), oligoacetal (>N—CH2[O—CH2]n—O—H), alkoxyoligoacetal (>N—CH2[O—CH2]n—OR), alkoxymethyl (>N—CH2—OR), methylene ether bridge (>N—CH2—O—CH2—N<), methylene bridge (>N—CH2—N<) and methylol (>N—CH2—OH) groups. The methylene ether bridge and methylene bridge link individual melamine units into oligomeric species which contain more than one triazine moiety in one molecule, which are usually referred to as dimers, trimers, etc.
Erikson et al. disclose in U.S. Pat. No. 3,322,762 the production of “substantially hexakis (methoxymethyl) melamine”. The synthesis thereof involves reacting a melamine molecule having six methylol groups per molecule with excess methanol, in this case, at least 20 moles of methanol per 1 mole of melamine, at a pH between about 1 and 3 to produce a “fully methylated” hexakis (methoxymethyl) melamine. The product can be dissolved in water in amount of up to a mass fraction of solids of 33% in water at 25° C.
Graziano et al. disclose in U.S. Pat. No. 5,376,504 the recovery of monomeric hexamethoxymethyl melamine from byproducts of synthesis by batch distillation of a commercial highly methylolated and highly etherified melamine formaldehyde resin, for use in preparing photoresists having improved sensitivity and shelf life.
As also oligomeric derivatives of alkoxymethyl melamine are useful crosslinking agents, there has been little incentive to develop processes for the manufacture of highly monomeric alkoxymethyl melamine products. Cameron et al. in U.S. Pat. No. 4,837,278 report the use of a “low imino, fully alkylated aminoplast crosslinking agent”. A commercial aminoplast resin is used in this patent, referred to as ®Resimene RF 4518 which, according to an analysis reported in US 2002/0 000 536 A1, is a mixed alkoxymethyl melamine comprising methoxy methyl and 2-ethylhexoxy methyl groups, typically comprising about 5 alkoxymethyl groups bound to one molecule of melamine, with a mass fraction of imino groups, >NH, of about 0.28%, and a mass fraction of monomers of about 30%.
An object of this invention is to provide improved alkoxymethyl melamine compositions for use as low temperature crosslinkers in the production of flexible and tough coating films.
Another object is to provide improved low temperature curing alkoxymethyl melamine compositions which provide coatings with improved solvent resistance. A still further object is to improve the cold storage stability of highly methylated liquid melamine-formaldehyde crosslinker resins as these tend to solidify or form solid deposit at prolonged storage at low temperatures. A still further object is to provide a basis to tailor the crosslinking efficiency according to the intended application.
These and other objects of the invention as will be apparent from the following detailed description and examples are achieved by providing mixtures of monomeric and oligomeric alkoxymethyl melamine compounds with high alkyl substitution and low imino content.